Volumetric Display

ABSTRACT

A three-dimensional image display device generates a virtual image within a defined imaging volume. The device includes a two-dimensional image display panel for generating a two-dimensional image; a first focusing element for projecting the two-dimensional image to a virtual image in an imaging volume; and means for altering the effective optical path length between the display panel and the projecting first focusing element so as to alter the position lens of the virtual image within the imaging volume. The effective optical path length may be varied by a lens of variable focal length, by relative motion of the 2D display a panel and the first focusing element, or by the introduction of other optical elements into the optical path that vary the effective optical path length.

The present invention relates to three-dimensional image displaydevices, and in particular to three-dimensional image display devicesthat generate a virtual image within a defined imaging volume.

A three-dimensional image can be created in several ways. For instance,in stereoscopic displays two pictures uniquely observable by each of aviewer's eyes can be shown simultaneously or time-multiplexed. Thepictures are selected by means of special spectacles or goggles worn bythe viewer. In the former case, the spectacles may be equipped withPolaroid lenses. In the latter case, the spectacles may be equipped withelectronically controlled shutters. These types of displays arerelatively simple to construct and have a is low data-rate. However, theuse of special viewing spectacles is inconvenient and the lack ofperspective may result in discomfort among viewers.

A more realistic three-dimensional impression can be created using anauto-stereoscopic display. In these types of display, every pixel emitslight with different intensities in different viewing directions. Thenumber of viewing directions should be sufficiently large that each ofthe viewer's eyes sees a different picture. These types of display showa realistic perspective; if the viewer's head moves, the view changesaccordingly.

Most of these types of display are technically difficult to realise inpractice. Several proposals can be found in the literature, see forinstance U.S. Pat. No. 5,969,850. The advantage of these displays isthat a number of viewers can watch, e.g. a single 3D television displaywithout special viewing spectacles and each viewer can see a realisticthree-dimensional picture including parallax and perspective.

Another type of 3D display is a volumetric display as described athttp://www.cs.berkley.edu/jfc/MURI/LC-display. In a volumetric display,points in an image display volume emit light. In this way, an image of athree dimensional object can be created. A disadvantage of thistechnique is occlusion, i.e. it is not possible to block the light ofpoints that are hidden by other objects. So, every object displayed istransparent. In principle, this problem can be overcome by means ofvideo-processing and possibly tracking of the position of the viewer'shead or eyes.

A known embodiment of a volumetric display is shown in FIG. 1. Thedisplay consists of a transparent crystal 10 in which two lasers 11, 12(or more) are scanning. At the position 15 of intersection of the laserbeams 13, 14, light 16 may be generated by up-conversion, where photonemission at a higher energy occurs by absorption of multiple photons oflower energy (i.e. from the combined laser beams). This type of displayis expensive and complicated. A special crystal 10 and two scanninglasers 11, 12 are required. In addition, up-conversion is not a veryefficient process.

An alternative embodiment of volumetric display 20 is shown in FIG. 2.This arrangement uses a material that can be switched betweentransparent and diffusive, such as polymer dispersed liquid crystal(PDLC) or liquid crystal gel (LC-gel). In a three-dimensional gridvolume 21, cells 22 can be switched between these two states. Typically,the volume 21 is illuminated from one direction. In the illustration,the illumination source 23 is located below the grid volume. If a cell22 is switched to a diffusive condition, light 24 is scattered in alldirections.

A still further type of display is described in WO 01/44858. Thisdocument describes a three-dimensional volumetric image display devicein which collimated light from an illumination source is incident upon aliquid crystal display panel that is superposed with a liquid crystalmicrolens array. Each microlens in the array is aligned with arespective pixel in the LCD panel to receive light therefrom. Eachliquid crystal microlens has an adjustable focal length so that lightfrom the respective pixel may be projected to a selected point in avolumetric image space. Thus, the light intensity and/or colour reachingeach microlens in the array may be controlled to produce a plurality ofcorresponding light intensities and colours in the volumetric imagespace.

A potential problem with this approach is that each LCD pixel has to bealigned with a respective microlens, and the separation between the LCDpanel and the microlens array is fixed in order to determine the depthof the volumetric image space. This results in a very limited viewingangle. In addition, the use of complex microlens arrays is required,together with a complex control system to separately control the focallength of each individual microlens element in the array.

It is an object of the present invention to provide a volumetricthree-dimensional image display device that overcomes some or all of theproblems associated with prior art devices.

According to one aspect, the present invention provides a display devicefor generating a three-dimensional volumetric image, comprising:

a two-dimensional image display panel for generating a two-dimensionalimage;

a first focusing element (42, 47) for projecting the two-dimensionalimage to a virtual image (40, 45) in an imaging volume (44, 49); and

means (43, 48, 50, 51, 60) for altering the effective optical pathlength between the display panel and the projecting first focusingelement so as to alter the position of the virtual image within theimaging volume.

According to another aspect, the present invention provides a method ofgenerating a three-dimensional volumetric image, comprising the stepsof:

generating a two-dimensional image on a two-dimensional image displaypanel (41, 46);

projecting the two-dimensional image to a virtual image (40, 45) in animaging volume (44, 49) with a first focusing element (42, 47); and

altering the effective optical path length between the display panel andthe projecting focusing element so as to vary the position of thevirtual image within the imaging volume.

Embodiments of the present invention will now be described by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 shows a perspective schematic view of a volumetric display basedon two scanning lasers and an up-conversion crystal;

FIG. 2 shows a perspective schematic view of a volumetric display basedon switchable cells of polymer dispersed liquid crystal or liquidcrystal gel;

FIG. 3 is a schematic diagram useful in explaining the principles of thepresent invention;

FIG. 4 is a schematic diagram illustrating volumetric three-dimensionalimage display devices comprising a display panel and a focusing elementaccording to embodiments of the present invention;

FIG. 5 is a schematic diagram of an arrangement for varying theeffective optical path length between the display panel and the focusingelement by way of two rotating cubes;

FIG. 6 is a schematic diagram of an arrangement for varying theeffective optical path length between the display panel and the focusingelement by way of a reflective rotating wheel; and

FIG. 7 is a schematic functional block diagram of a control system forthe display device of FIG. 4.

FIGS. 3 a and 3 b illustrate some basic principles used in the presentinvention. In FIG. 3 a, a relatively large virtual image 30 of a smalldisplay panel 31 is provided by a Fresnel mirror 32. In FIG. 3 b, arelatively large virtual image 35 of a small display panel 36 isprovided by a Fresnel lens 37. The virtual image 30 or 35 appears in theair in front of the lens. A spectator can focus on this image 30 or 35and observes that it is ‘floating’ in the air.

FIGS. 4 a and 4 b illustrate a modification to the arrangements of FIGS.3 a and 3 b, according to the present invention. As shown in FIG. 4 a,the effective optical path length between the display panel 41 and theFresnel mirror 42 is varied by the provision of a dynamic lens 43.Similarly, as shown in FIG. 4 b, the effective optical path lengthbetween the display panel 46 and the Fresnel lens 47 is varied by theprovision of a dynamic lens 48.

The dynamic lens 43 or 48 has a dynamically adjustable optical strength.By weakening the optical strength of this lens, the virtual image 40 or45 will shift away from the Fresnel lens or mirror 42 or 47. If theadjustable lens 43 or 48 is made stronger, the virtual image 40 or 45will shift towards the Fresnel lens or mirror. It is noted that theeffect of increasing or decreasing the optical power of the dynamicallyadjustable lens 43 or 48 is to vary the effective optical path lengthbetween the display panel 41 or 46 and the to Fresnel lens or mirror, byvirtue of localised changes in refractive index within the optical path.

In a general sense, it will be noted that the mirror 42 or lens 47 maygenerally be replaced or implemented by any optical focusing element forprojecting the two dimensional image of the display panel 41, 46 to avirtual image 40 or 45 located within an imaging volume 44 or 49.Preferably, the mirror 42 or lens 47 is a single or compound opticalfocusing element having a single focal length such that a planar displaypanel is imaged into a single plane of an-imaging volume.

In operation, the optical strength of adjustable lens 43, 48 or, moregenerally, the effective optical path length between the two-dimensionaldisplay panel 41 or 46 and the focusing element 42 or 47, is adjustedperiodically at a 3D image display frame frequency. Typically this wouldbe 50 or 60 Hz. So, during one 3D image frame period (e.g. 1/50 sec),the virtual image of the display panel 41 or 46 fills the imaging volume44 or 49. Within the same frame period, the display panel may be drivento alter the image that is projected, so that different depths withinthe imaging volume 44 or 49 receive different virtual images.

It will be understood that in a preferred aspect, the means for alteringthe effective optical path length between the 2D display panel 41 or 46and the focusing element 42 or 47 is effective to periodically sweep asubstantially planar virtual image of the substantially planar twodimensional display panel through the imaging volume 44 or 49 at a 3Dframe rate. Within that 3D frame period, the 2D image display paneldisplays a succession of 2D images at a 2D frame rate substantiallyhigher than the 3D frame rate.

Therefore, at different planes 40 a, 40 b or 45 a, 45 b in the imagingvolume 40, 45, different images are obtained so that a three-dimensionalimage of any object can be constructed.

The two-dimensional display panel may be any suitable display device forcreating a two dimensional image. For example, this could be a poly-LEDdisplay or a projection display based on a digital micromirror device(DMD).

Preferably, the display panel is sufficiently fast to enable thegeneration of plural 2D images within one frame period of, e.g. 1/50sec. For example, commercially available DMDs can reach speeds of 10,000frames per second. If 24 two-dimensional frames are used to createcolour and grey-scale effects and a 3D image refresh rate of 50 Hz isrequired, it is possible to create eight different image planes 40 a, 40b, 45 a, 45 b in the imaging volume 44, 49 is. The dynamicallyadjustable lens could be any suitable device such as a liquid crystaladaptive lens, a deformable lens (e.g. deformable electrically,thermally or mechanically), or could be substituted for by a deformablemirror system. Preferably, the dynamically adjustable lens is a singleor compound lens having a substantially constant focal length over itsentire working area, albeit adjustable focal length. The working area ofthe focusing element should be sufficiently large to image the entireworking display area of the display panel.

In the case of a liquid crystal adaptive lens, this may be achieved witha sheet of material whose refractive index properties can vary as afunction of applied electric field. An array of transparent electrodesare provided adjacent to the surface of the sheet, and these are used tolocally control the refractive index so that it varies spatially acrossthe sheet and thereby forms a focusing lens of selected focal length. Inthis embodiment, it will be understood that the effective focal lengthis varied by electro-optic control.

In the case of a deformable lens or mirror, this may be achieved by anelastic or plastic material of suitable refractive index whose shape maybe distorted in order to provide a lens or mirror of selected focallength. In these embodiments, it will be understood that the effectivefocal length of the focusing element is varied by mechanical means, e.g.by electromechanical, magneto-mechanical or acoustic transducer.

In another embodiment, alteration of the effective length of the opticalpath between the display panel 41, 46 and the focusing element 42, 47 isachieved by varying the physical path length, as well as or instead ofvariation in the effective optical path length by refractive indexadjustments as already discussed.

Adjustment of the physical distance between the display panel 41, 46 andthe focusing element 42, 47 may be achieved mechanically, simply byphysical movement of one or the other (or both) of the display panel andthe focusing element (i.e. by alteration of their relative positions).This can be by way of a suitable motor drive or vibration mechanism.

FIG. 5 shows an alternative technique for altering the physical path islength. In FIG. 5 a, two rotating cubes 50, 51 are positioned in theoptical path between the display panel 46 and the focusing element 47.When the two cubes 50, 51 have faces orthogonal to the light path 52,the optical path is undiverted. When the two cubes 50, 51 arecontra-rotated slightly as shown in FIG. 5 b, a portion 53 of theoptical path 52 is diverted slightly downwards as shown. The two cubesare contra-rotated in synchronism such that the light ray leaves thesystem along the same path. Due to the parallel displacement of theportion 53 of optical path 52 in between the two cubes, the opticaldistance between the display and the lens can be altered.

In a general sense, it will be understood that the two rotating cubesoperate as a pair of displaceable refraction elements for displacing andthereby varying the length of a portion of the optical path.

Another alternative is described in connection with FIG. 6. A segmentedwheel 60 has a thickness that varies from segment to segment, e.g.segments 61-64. If the two-dimensional display is imaged via or throughthe wheel 60, the effective optical path length changes with rotation ofthe segmented wheel. The 2D display panel 46 is positioned to one sideof the main optical path alignment and light therefrom is deflected byhalf-mirror 65 to the reflective rotating wheel 60. The rotating wheelis driven by a motor 66 and may be configured in several different waysto effect a change in effective path length as a function of rotation.

In a first configuration, the rotating wheel has a reflective uppersurface. In this way, light incident on the wheel reflects off an uppersurface which is effectively changing in height as the wheel rotatesthereby shortening optical path section 67. This affects both theinbound and reflected light beam. The light reflects back to thehalf-mirror 65 and then continues to the focusing element 47 to formvirtual image 45.

In a second configuration, the rotating wheel has a reflective lowersurface. In this way, light incident upon the wheel 60 travels throughthe thickness of the segment 61-64 upon which it is incident and thenreflects back to the half-mirror 65. The light then continues to thefocusing element 47 to form virtual image 45. In this situation, theeffective change in optical path length is altered by the introductionof varying thickness of optical material corresponding to each segment61-64, each having a higher refractive index than the free space path.

In a third configuration, the same effect could be achieved by thesegmented wheel being of constant thickness but with segments of varyingrefractive index.

Preferably, the optical system comprising the focusing element 47enlarges the image of the 2D display device. In such a case, only asmall adjustment of the distance between the 2D display and the lensresults in a large displacement of the virtual image. Let us denote thedistance between the 2D display and the lens by o and the distancebetween the lens and the virtual image by b. Then, there holds thefollowing relation between the lens strength a[m], o[m] and b[m]:

1/f=1/o+1/b

An increase of the object distance by Δo results in an increase of thedistance b by Δb:

Δb=−M ² Δo

where M=b/o is the magnification factor. Since M is larger than 1,typically, between 5 and 10, a small increase in o results in a largedisplacement of the virtual image.

The volumetric displays as described herein can generally be simple toconstruct and can be assembled from well known parts. Applications forsuch volumetric displays are widespread, including in the professionalmarket, e.g. CAD/CAM and medical applications, and in the domesticmarket, e.g. for entertainment devices.

With reference to FIG. 7 a schematic view of an overall volumetric imagedisplay device with control system is shown. The effective optical pathlength modifier 70 a (such as adaptive lens 43, 48, rotating cubes 50,51 or segmented wheel 60) interposed between the 2D display panel 46 andfocusing element 47 is controlled by path length control circuit 73.Alternatively, a motorised stage 70 b for varying the position of the 2Ddisplay panel 46 is controlled by the path length control circuit. Adisplay driver 72 receives 2D frame image data from image generator 71.Display of the succession of 2D images is synchronised with the pathlength controller operation by way of a synchronisation circuit 74.

Other embodiments are intentionally within the scope of the accompanyingclaims.

1. A display device for generating a three-dimensional volumetric image,comprising: a two-dimensional image display panel (41, 46) forgenerating a two-dimensional image; a first focusing element (42, 47)for projecting the two-dimensional image to a virtual image (40, 45) inan imaging volume (44, 49); and means (43, 48, 50, 51, 60) for alteringthe effective optical path length between the display panel and theprojecting first focusing element so as to alter the position of thevirtual image within the imaging volume.
 2. The display device of claim1 in which the means (43, 48, 50, 51, 60) for altering the effectiveoptical path length is adapted to operate so as to move the virtualimage periodically through the imaging volume.
 3. The display device ofclaim 2 further including: a display driver (72) for controlling thedisplay panel to generate a succession of different images as thevirtual images corresponding thereto are moved through the imagingvolume; and control means (73, 74) for synchronising the display driverwith the means (70) for altering the effective path length.
 4. Thedisplay device of claim 1 in which the means for altering the effectiveoptical path length comprises a second focusing element (43, 48) havingadjustable optical strength.
 5. The display device of claim 4 in whichthe second focusing element (43, 48) is a liquid crystal adaptive lens.6. The display device of claim 4 in which the second focusing element(43, 48) is a deformable lens.
 7. The display device of claim 4 in whichthe second focusing element (43, 48) is a deformable mirror.
 8. Thedisplay device of claim 1 in which the means for altering the effectiveoptical path length comprises physical displacement means (70 b) foraltering the relative positions of the display panel (46) and the firstfocusing element (42, 47).
 9. The display device of claim 1 in which themeans for altering the effective optical path length comprises anoptical path modifier (50, 51, 60) for varying at least a portion of theoptical path between the display panel (46) and the first focusingelement (42, 47).
 10. The display device of claim 9 in which the opticalpath modifier (50, 51) is adapted to vary a distance travelled by thelight from the display panel (46) to the focusing element (42, 47). 11.The display device of claim 9 in which the optical path modifier (50,51, 60) is adapted to vary the refractive index of at least a portion ofthe optical path.
 12. The display device of claim 10 in which theoptical path modifier comprises a plurality of displaceable refractionelements (50, 51) for displacing and thereby varying the length of aportion (53) of the optical path.
 13. The display device of claim 10 inwhich the optical path modifier is a reflective element (60) having aplurality of portions of differing height which may be selected toreflect the light from the display panel to the focusing element fromdifferent physical locations; and selection means (66) for varying theportion of optical element that lies in the optical path.
 14. Thedisplay device of claim 11 in which the optical path modifier (60)comprises: an optical element (60) between the display panel (46) andthe first focusing element (47), having a different refractive indexthan other parts of the optical path between the display panel and thefirst focusing element, the optical element having portions (60-64) ofvarying thickness and/or refractive index; and selection means (66) forvarying the portion of optical element that lies in the optical path.15. The display device of claim 1 in which the first focusing element(42, 47) enlarges the virtual image (40, 45) of the display panel (41,46).
 16. The display device of claim 1 in which the first focusingelement (42, 47) projects the two dimensional image into a correspondingplanar virtual image (40, 45) in the imaging volume (44, 49), the meansfor altering the effective path length altering the distance of theplanar virtual image from the optical output of the first focusingelement (42, 47).
 17. The display device of claim 1 in which the displaypanel (41, 46) is adapted to have an image refresh rate substantiallygreater than 50 frames per second.
 18. The display device of claim 1 inwhich the display panel (41, 46) is adapted to have an image refreshrate greater than 200 frames per second.
 19. The display device of claim1, in which the means for altering the effective optical path lengthincludes mechanical means.
 20. The display device of claim 1, in whichthe means for altering the effective optical path length includeselectro-optical means.
 21. The display device of claim 1 in which thefirst focusing element is a single or compound element havingsubstantially a single focal length.
 22. The display device of claim 4in which the second focusing element is an adjustable, single orcompound lens having a substantially constant focal length over itsentire working area.
 23. A method of generating a three-dimensionalvolumetric image, comprising the steps of: generating a two-dimensionalimage on a two-dimensional image display panel (41, 46); projecting thetwo-dimensional image to a virtual image (40, 45) in an imaging volume(44, 49) with a first focusing element (42, 47); and altering theeffective optical path length between the display panel and theprojecting focusing element so as to vary the position of the virtualimage within the imaging volume.
 24. The method of claim 23 includingmoving the virtual image periodically through the imaging volume. 25.The method of claim 24 further including the steps of: controlling thedisplay panel to generate a succession of different images as thevirtual images corresponding thereto are moved through the imagingvolume; and synchronising the images of the display panel with theperiodic movement of the virtual images through the display volume. 26.The method of claim 23 in which the step of altering the effectiveoptical path length comprises varying the optical strength of a secondfocusing element (43, 48).
 27. The method of claim 23 in which the stepof altering the effective optical path length comprises altering therelative positions of the display panel (46) and the first focusingelement (42, 47).
 28. The method of claim 23 in which the step ofaltering the effective optical path length comprises varying at least aportion of the optical path between the display panel (46) and the firstfocusing element (42, 47).
 29. The method of claim 28 further comprisingvarying a distance travelled by the light from the display panel (46) tothe focusing element (42, 47).
 30. The method of claim 28 furthercomprising varying the refractive index of at least a portion of theoptical path.
 31. The method of claim 29 further comprising varying thepositions of a plurality of displaceable refraction elements (50, 51)for displacing and thereby varying the length of a portion (53) of theoptical path.
 32. The method of claim 29 further comprising the step ofintroducing a succession of mirrors into the optical path, each mirrorhaving a different position with respect to an incident optical beam.33. The method of claim 30 further comprising altering the position ofan optical element (60) between the display panel (46) and the firstfocusing element (47), having a different refractive index than otherparts of the optical path between the display panel and the firstfocusing element, the optical element having portions (60-64) of varyingthickness and/or refractive index.
 34. The method of claim 23 furtherincluding the step of refreshing the image in the display panel at arefresh rate substantially greater than 50 frames per second.
 35. Themethod of claim 34 in which the step of refreshing the image in thedisplay panel is at a refresh rate greater than 200 frames per second.